1. Field of the Invention
The invention relates to holographic storage, in particular to holographic storage processes such as phase correlation multiplexing.
2. Description of the Related Art
Holography involve s a process by which an image is stored as an interference pattern formed in a storage medium by the interference between a signal beam representing the image and a reference beam, and conversely, holography involves the process by which images are reconstructed from such interference patterns.
More specifically, holographic memory systems involve the three-dimensional storage of holographic representations (i.e., holograms) of data elements as a pattern of varying refractive index and/or absorption imprinted into the volume of a storage medium. Holographic memory systems are characterized by their high density storage potential and the potential speed with which the stored data is randomly accessed and transferred.
In general, holographic storage memory systems operate by combining a data-encoded object beam with a reference beam to create an interference pattern in a photosensitive storage medium. (See, e.g., Holographic Data Storage, by H. J. Coufal, D. Psaltis, G. T. Sincerbox (Editors), Springer-Verlag, New York, 2000, P.3-59, which is hereby incorporated by reference.) The interference pattern induces material alternations in the medium that create a hologram. The formation of the hologram in the storage medium is a function of the relative amplitudes and polarization states of, and phase differences between, the object beam and the reference beam. It is also highly dependent on the incident beams"" wavelengths and the angles at which the object beam and the reference beam are projected into the storage medium.
Holographically stored data is reconstructed by projecting into the medium a reference beam at the same angle, wavelength, phase, and position as the reference beam used to produce the hologram. The hologram and the reference beam interact to reconstruct the stored object beam. The reconstructed object beam then is detected, e.g., using a photodetector array, and the recovered data is post-processed for delivery to output devices.
Typically, the dynamic range of the holographic storage medium is larger than what is needed to store a single hologram with an acceptable signal-to-noise ratio. Therefore, it is often desirable to multiplex a number of holograms at one location to attain greater storage density. One technique for multiplexing is phase correlation multiplexing, as discussed in U.S. Pat. No. 5,719,691, the disclosure of which is hereby incorporated by reference. In phase correlation multiplexing, correlation is used to differentiate overlapping holograms within a medium. Correlation selectivity relies on the differences in amplitude, phase, and angle content of the reference beam generated by the relative shift of the storage medium with respect to the reference beam.
Basically, correlation multiplexing involves the storage of multiple holograms in an overlapping fashion. These holograms can be individually accessed through small translations of the storage medium relative to an object beam and a reference beam. The reference beam is spatially modulated with hologram position information. This highly complex reference beam relies on the phase, amplitude and angle differences induced by translating the medium relative to the complex reference for selectivity. A stored pattern of a single hologram can be recovered when the read-out reference beam is centered on the stored image to an accuracy determined by the by the correlation function of the reference beam (which in practice can be as short as a few microns).
Phase distance multiplex holography is described in U.S. Pat. No. 5,943,145, the disclosure of which is hereby incorporated by reference. In phase distance multiplexing, selection among multiple overlapping holograms within a medium is achieved by changing the distance between the medium and a phase mask used to impart phase content to a reference beam. It involves relative movement of the medium and the reference beam.
The use of a reference beam having correlated phase content during correlation multiplexing is disclosed in U.S. Pat. No. 6,191,875, the disclosure of which is hereby incorporated by this reference. It was found that reference beams having random phase content limited the attainable storage density and signal to noise ratio, (SNR) for stored holograms due to uncontrolled fluctuations in the correlation selectivity. A reference beam having correlated phase content provides higher SNR and the ability to tailor the reference beam spectrum to minimize cross talk between neighboring holograms.
A practical consideration in commercial holographic storage devices is simplification of the movements used in accessing individual holograms in an array of overlapping holograms. For instance, in one system in which the holographic storage medium has a disk format, accessing a stack of holograms involves sliding at least a portion of a lens system to different radial positions above the disk. Once the lens system portion is in a given radial location, a holographic storage location associated with the given radial location can be accessed for recording an image as a hologram at the location or for reconstructing an image previously recorded as a hologram at the location. A particular problem in a phase correlation multiplexing system is to simplify the small movements required to change the reference beam phase structure sufficiently to access different overlapping holograms. For example, a movement of 30 microns with 1/10-micron accuracy may be required to access successive overlapping holograms. In general, the more massive the object to be moved, the more difficult it can be to achieve such small high precision movements.
Thus, there has been a need for improvements that simplify the movements required to change a reference beam structure so as to access different overlapping holograms located in about the same location of a storage medium. The present invention meets that need.
One aspect of the present invention involves rotation correlation multiplex (RCM) holography in which an array of overlapping holograms can be recorded or can be reconstructed in a holographic storage medium. The holograms are produced by interference of a reference beam and a signal beam in which the reference beam is a phase beam, which consists essentially of a multitude of rays of varying angle of incidence and non-uniform phase. Holography in accordance with the invention involves changing the orientation of a complex phase beam relative to the medium. In one embodiment, changing the orientation of the complex phase beam involves rotation of a phase mask so that the structure of the phase beam incident on the medium is changed sufficiently to select one of many different holograms stored at a given medium location.